Check Valve

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I am having a sprinkler system installed and the company says I need an anti-siphon valve. They explained what it does and why I need it and it all makes sense. I asked why not just put a check valve in to prevent any backflow. They just said that the law says you have to have a anti-siphon valve. So does the anti-siphon valve do something that the check valve can't? Thanks for your help.

Both do prevent the revers flow of water in a plumbing system, but a anti-siphon also breaks the water seal in the line. Same as using a drinking straw and you but your thumb over the end. The fluid is still in the straw, but once you release your thumb the fluid will clear out. Most anti-siphon valves are to be installed 12" higher then any other opening in the system to allow the water to drain out. Inside the valve is a diaphragm that drops with negative water pressure and seals the water supply, a atmospheric opening occurs when the diaphragm drops allowing air to enter the valve, therefore breaking the water seal.